Q1. At the moment is the anti-crash system effective? How does it work? Turns off power to sections of rail until the next section has no current draw? This could impact suggested solutions.
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So is there a practical/political reason why the 'loop' section cannot use PWM too? PWM means all locomotives can be controlled and no locomotives require modification. Hence locomotives can be ported to other layouts without the need for removing the mod and, [non DCC] locomotives can guest on the layout.
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The loop is using PCB's from the 80's (from a company called Trans Electronic Railways, made in Belgium
). The whole loop is cut into smaller sections which are each controlled by one of these PCB's.
I sketched a simplified schematic of the circuitry that controls the voltage on the track and does the detection of the train in the block (see attachment 610_Basic1.jpg).
The voltage on the track is a pure DC voltage, set by an NPN pass device. A PTC thermistor is added for short-circuit protection.
The detection (drawn in blue) is sensing the voltage drop over a diode. Based on this detection the PCB can tell the previous block to put the signal to red and stop any train entering the next block.
When a train is stopped in a block there is still a small voltage (~1V) applied to make the detection circuit work.
Changing the PCB's to work with PWM would make things more complicated. E.g. the detection circuit would also need to be changed. Also the PCB's are working fine so it's not really needed.
Also that wouldn't solve the problem because the PCB's can't recognise which train is currently in the block to set the appropriate voltage. Adding that would mean we end up with something that's pretty much equal to digital (DCC) control.
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Q2. Approximately how mismatched are the train speeds when fed a normal voltage? Eg magnitude of 2:1, 5:1, 10:1?
Q3: Approximately how matched do you want to make them? Eg within 2:1, 1.5:1?
My estimation is a mismatch of maximum 2:1 between the fastest running train and a normal running train. The target would be to make them approximately equal.
Have you tried adding incandescent light bulbs in series with the motor?
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No, I haven't. But it's a good idea. I think it works very similar to adding a PTC thermistor in series to limit the current, right?
Some random thoughts about a circuit to put in each train to make the max speed (voltage) adjustable:
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Wouldn't a simple voltage divider and emitter follower work ok here?
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I also had an idea in this direction: see attachment VoltageClamp.jpg. But I used a depletion FET to limit the deadband around 0V.
It also works like a voltage clamp: For voltages lower than the zener voltage, the FET would have a small voltage drop. For larger voltage the motor voltage would be clamped to something around the zener voltage.
Power dissipation can indeed be an issue. I need to do some further research to see if it's possible without adding an enormous heatsink in the locomotive.
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Alternatively, you can design a small DC motor controller that reads the RPM of the motor, and define whatever speed vs voltage curve is needed. Much harder and more expensive, but with predictable and reproducible results.
That idea crossed my mind, but it looks that this will get complicated quickly. Some tricky things that I can tell from the top of my head: powering the controller (rectifier + capacitors?), sensing PWM versus DC inputs, sensing forward/backward direction, driving the motor when PWM is applied.
But it is an option to consider.
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Alternatively, is there a mechanical solution whereby the gearing is changed to spin the wheels slower for the same motor speed? That would give the sets a higher torque.
That's a good idea! I will discuss it with the other members of the association.